Fixed #7312 -- Fixed handling of custom Q-like objects in QuerySet.custom_filter().

This is pretty much internal-use-only code, so doesn't affect public API at
all, but it's nice to be able to handle things properly in any case. Patch from
emulbreh.


git-svn-id: http://code.djangoproject.com/svn/django/trunk@7766 bcc190cf-cafb-0310-a4f2-bffc1f526a37